1. Appealing to an older demographic from the start, which set the tone. 20s and 30s (and up) are more common here than average. A younger crowd was brought in by Valley 1 and so forth, but the tone was already set.
2. Appealing to a highly intelligent/educated demographic from the start, which set the style for grammar and so on.
3. Setting personal examples by trying to set emotion aside and mostly succeeding. Eating crow and accepting mistakes rather than trying to hide them, and so on. Typing properly ourselves. All of these things help to set and reinforce the tone.
4. Actively moderating people who are abusive, which happens rarely, and giving them warnings or even temporary bans. We haven't had to do that in over a year, I don't believe, and it's been less than 10 times overall in the life of the forums, I think. This cuts down on the language and so forth, though.
5. Posting a clear set of rules, with the reminder in them that tone is not conveyed through the written word very well. This seems minor, but honestly seemed to help as we had fewer temp-bans and incidents after this than before this.
6. Treating new folks with enthusiasm and respect, and generally not showing favoritism in any way to people based on post count. This again is something that the community then tends to emulate, as the tone of welcoming newcomers really is a positive thing. More likeminded souls for them to communicate with, a larger community, etc.
7. Having a small community, particularly at first. Things have shifted each time we've had a major growth period, and it's gotten a little less personal and has had more problems. I don't know what the breaking point is past which this style of community is no longer possible, but there is such a point I am quite certain. Hopefully we never reach it.
8. Actively enlisting players in helping to improve the games we make, and giving them a real voice. This is huge, because it shows literal respect from us to them. It binds the community together, as well as giving a common purpose. It helps take the emotion out of things on the player side, because people don't rail on us in futile frustration: they say something, we and the community listen and either have a back and forth, or archive it for later, or implement it soon or now. Even if their suggestion/complaint/whatever is not addressed, they know it's been read and fairly considered. It didn't just go into a black hole, or have some snarky phone operator trying to shout them down.
9. Giving generously. Our habit of doing lots of patches and so forth that are more than just bugfixes, plus doing really-excellent-value expansions, again sets a tone. Players are then generous with us in return, both in terms of time spent helping improve the game, money spent buying the game for friends and so forth, and effort spent telling people about the game in other places. It becomes symbiotic, because it's clear that we're not greedy and so players don't resent us for that either.
10. Being open about the state of the company, and asking for help when we need it. Pointing out that if players want more of the sort of stuff we create, we need to stay in business and that we need their help to do that. This gives players a sense of investment and of being a part of things, and also helps to make the addition of "new blood" into the forums a source of joy, rather than "ugh newbies."
11. And finally, creating the sort of games that foster creativity and deeper thought. When someone comes and asks a "noob" question, people don't scoff: they're proud to share their knowledge, or their strategies, or both. It's a mark of pride that they know the answer, and they want to help share that knowledge with others. There's enough going on that players don't feel like they have to hoard their best strategies or they fall behind. Part of that might also come from the fact that all our games are solo and co-op, not PVP. So there's less inherent antagonism right from the start.
TLDR: It's kind of a perfect storm of things, really. In a positive sense. Some of these things were deliberately done on our part in order to make a safe, positive place. Other things just kind of unexpectedly happened, and still other things were byproducts of decisions that we made that were wholly unrelated to this topic (such as the kind of games we make). It's a topic I've thought a lot about, actually.